1. Field of the Invention
The present invention, in particular, relates to a water tube boiler capable of reducing CO emission and a method for controlling combustion in the same.
2. Description of the Prior Art
A water tube boiler has been known which comprises a combustion chamber, a group of water tubes extending therethrough, and a burner for heating water passing through the group of water tubes by combustion gas to obtain hot water or steam. In such a water tube boiler, a large combustion zone is provided between the burner and the group of water tubes to sufficiently advance oxidation of CO into CO.sub.2, and then heat exchange is effected between the combustion gas and the group of the water tubes disposed on further down stream from the combustion zone. This is because if heat exchange was carried out prior to sufficient oxidation of CO contained in the combustion gas, an exhaust gas would contain residual CO in an undesirable amount. Accordingly, the volume of the combustion chamber is unavoidably large.
To cope therewith, as a combustion method and an apparatus for carrying out the method which are capable of achieving low CO-generating combustion with a combustion chamber having a reduced size, there have been proposed those in which water tubes are placed in the vicinity of the tip of a flame to control the temperature of the flame within a range of about 1,000.degree. C. to 1,500.degree. C., and then the temperature-controlled combustion gas is caused to pass through an adiabatic space and then led into a heat exchanger to effect heat exchange, thereby realizing high intensity combustion while reducing CO emission (Japanese Examined Patent Publication No. 35884/1990). The method and the apparatus have been made in view of the fact that oxidation reaction of CO into CO.sub.2 is promoted at a temperature within the range of about 1,000.degree. C. to 1,500.degree. C. The temperature of the flame is decreased in the first step by means of water tubes, and then CO is oxidized in the adiabatic space. It is thereby possible to realize a combustion chamber as a whole several times as small as a conventional combustion chamber and also realize low CO-generating combustion.
The method and the apparatus effectively function as means for achieving low NOx/low CO-generating combustion when used in a water tube boiler utilizing a burner capable of producing flat flames which are uniform almost throughout a section of a combustion chamber and develop two-dimensional flows (see, for example, Japanese Unexamined Patent Publication No. 229608/1995). However, it is required for realizing such low NOx/CO-generating combustion and for effectively utilizing the adiabatic space to generate flat flames spreading over the section of the combustion chamber. Further, a premixed combustion system is often used to reduce a size of a combustion chamber. However, the pre-mixer is required to generate a premixture, and a safety device peculiar to premixed combustion is required to prevent back fire, explosion or the like. These result in a very complicated structure of the combustion apparatus. Accordingly, it is desired to use a diffusion flame burner having a relatively simple structure in terms of reduction in a cost of the burner, and ease in manufacture and maintenance of a burner. It is also desired to attain low CO-generating combustion in a water tube boiler using a diffusion flame burner.
The present inventors have conducted various combustion experiments using diffusion flame burners to solve the above problems, and through the experiments, they have experienced that if use is made of, in a water tube boiler, a burner whose burner ports are located only at such positions as to provide jet flames each localized in a cross-section of a combustion chamber of the water tube boiler, the jet flames are rapidly cooled by water tubes to prevent CO oxidation reaction from satisfactorily proceeding, and accordingly, the size of the combustion chamber is unavoidably large so as to surely provide a sufficient combustion zone.
It is, therefore, an object of the present invention to provide a water tube boiler of such a type that it uses a diffusion flame burner with burner ports located only at particular positions in a burner mounting wall of a combustion chamber to generate jet flames, which has a novel structure capable of realizing a size-reduced combustion chamber and capable of continuously effecting stable low CO-generating combustion.
To solve the above problem, the present inventors have conducted combustion experiments using a water tube boiler as schematically shown in plan in FIG. 8. In FIG. 8, reference number 1 represents a combustion chamber having a rectangular section. A burner 2 is mounted in one end la of the combustion chamber 1, and a multiplicity of water tubes 3 are inserted through the combustion chamber 1 in the proximity of the burner 2.
FIG. 9 is an enlarged sectional view showing the burner 2 and a portion of the combustion chamber 1 in the vicinity thereof. FIG. 10 is a sectional view taken along the line X--X in FIG. 9. FIG. 11 is a fragmentary enlarged sectional view of the burner 2 taken along the line XI--XI in FIG. 10. As shown in FIG. 9, the burner 2 comprises a burner casing 21, an air pipe 22 having an inner diameter of D and disposed in the burner casing 21, a fuel pipe 10 inserted through the air pipe 22 coaxially therewith, a shielding plate 23 which contacts with the inside surface of the air pipe 22 and through which the fuel pipe 10 extends into the combustion chamber 1 having a section of 2D square, and a circular plate 24 attached to the front end of the fuel pipe 10.
As shown in FIG. 10, the shielding plate 23 are provided with four slot-like portions 25 at angular intervals of 90 degrees, each of which functions as an air injection portion. The fuel pipe 10 is provided with four fuel injection nozzles 26 adjacent to the shielding plate 23 which extend toward proximal edges of the slot-like portions 25. The tips 27 of the injection nozzles 26 are located in close vicinity of the proximal edges of the respective slot-like portions 25. The circular plate 24 has such a size that it is substantially inscribed in the proximal edges of the slot-like portions 25 when viewed in the axial direction of the fuel pipe 10. As shown in FIG. 10, the burner 2 constructed as described above is mounted on the combustion chamber 1 in such a manner that the slot-like portions 25 formed in the shielding plate 23 are arranged on the diagonal lines of the 2D square cross-section of the combustion chamber 1. Incidentally, the burner 2 is described in detail in Japanese Patent Application No. 106878 filed in the name of the applicant who is the same entity as the assignee (applicant) of the present application.
In the water tube boiler constructed as described above and used in the experiments, air from the air pipe 22 and a fuel gas from the fuel injection nozzle 26 are supplied from the four slot-like portions 25 adjacent to the one end 1a of the combustion chamber 1, and the fuel gas initiates combustion forming flame 4 not stabilized at the slot-like portions 25 generating through the group of water tubes 3 to effect heat exchange between the flame 4 and the water tubes 3.
The burner 2 was mounted on the water tube boiler at a distance L from the water tubes 3, i.e., at a standard position. The experiment was conducted using the burner 2 at a combustion rate of 28.times.10.sup.4 kcal/h, and CO content of the resulting exhaust gas was measured. Then, with a combustion chamber which was altered so as to prolong the distance between the burner 2 and the water tubes 3 by about 60%, i.e., from L to 1.6 L, the same experiment was conducted. In this connection, the prolonged amount corresponds to about 20% of the visible flame length. The results are shown in FIG. 12.
As understood from FIG. 12, when combustion was carried out with the burner located at the standard position to expose the group of water tubes to the jet flames running therethrough, oxidation reaction of CO was retarded due to the jet flames being rapidly cooled by the water tubes. In consequence, CO emission was in excess of 200 ppm. On the other hand, when the distance between the burner and the water tubes was prolonged from L to 1.6 L to provide a longer combustion reaction zone on the upper side of the water tubes, CO emission was somewhat moderated but still not reduced sufficiently (to a level of 100 ppm or lower). To further reduce CO emission in this mode, a still larger combustion zone is required. This results in a undesirably large-sized boiler as a whole. Accordingly, it has experimentally been proved that the mode where a larger CO oxidation reaction zone is provided on the upper side of water tubes cannot suitably be employed with a view to obtaining a compact combustion chamber of a water tube boiler using jet flames.
Thereupon, the present inventor has further proceeded with experimental researches and consequently found that not by the mode where a prolonged CO oxidation reaction zone is provided on the upper side of water tubes but by providing a space empty of water tubes in the domain of a group of water tubes, which extends through the domain in the direction of jet flames, CO content of an exhaust gas is greatly reduced without enlarging a combustion chamber. The present invention has been made on the basis of the finding.